The present invention relates to an image reader and a method for controlling the image reader. More particularly, the invention relates to an image reader capable of reading an image and a translucent original such as photographic film at high quality through self-advancement of a read unit provided with a reducing optical system employing an image-forming lens, as well as to a method for controlling the image reader.
Conventionally, a flat-bed-type image (or original) reader for reading an image through self-advancement of a read unit provided with a reducing optical system employing an image-forming lens is known as an image reader or an original reader (i.e., scanner) for reading, as image information, character information and graphic information present on a medium such as paper.
FIG. 18 shows a conventional flat-bed-type image reader. As shown in FIGS. 18A and 18B, the flat-bed-type image reader is composed of a flat bed unit 151 and a read unit 152.
The flat bed unit 151 has an original bed 161 for placement of an original, a guide rail 162, and a reference shaft 171. The read unit 152 has a reducing optical system and a contact member 153. The reducing optical system includes a fight source 175, reflecting mirrors 176, an image-forming lens 177, and a CCD 178. The contact member 153 includes a sliding member or a rolling member displaced so as to face the guide rail 162.
When an original is scanned, the read unit 152 is driven by an unillustrated driving belt and caused to travel on the reference shaft 171. A side of the read unit 152 which faces the guide rail 162 slides on a surface of the guide rail 162. The surface of the guide rail 162 is finished to a high degree of flatness.
Accordingly, in the conventional image reader of FIG. 18, a positional error S may arise between the original bed 161 and the read unit 152, as shown in FIG. 18C, due to various factors, such as the flatness and attachment accuracy of the guide rail 162 and the distortion of the entire image reader, including the flat bed unit 151 and the read unit 152. When the error xcex4 arises, the optical path length between the original bed 161 and the CCD 178 becomes unstable.
Meanwhile, in recent image readers, the read unit 152 provided with a reducing optical system employing an image-forming lens has implemented higher resolution. In order to implement higher resolution, as shown in FIG. 19, the image-forming lens is forced to employ a shallow depth of field and a shallow depth of focus.
Accordingly, the conventional image reader involves the following problems.
(1) An unstable optical path length causes impairment in reading accuracy.
(2) Defocus results from a failure of an image to fall within the depth of field.
(3) Distortion of the entire image reader brings about a positional shift (skew) between a read start position and a read end position.
In an image reader (scanner) having a function to read a translucent original made of, for example, a transparent film, a light source for irradiating an original with light and a light-receiving sensor for receiving image-representing light which has passed through the original are moved in a mutually facing manner, thereby reading image information from the original. In order to read image information of the translucent original at high quality, an appropriate positional relation must be maintained between the light source for irradiating the original with light and the light-receiving sensor for receiving image-representing light which has passed through the original.
FIG. 20 shows a conventional flat-bed-type image reader capable of reading a translucent original 201. As shown in FIG. 20, the image reader is composed of a reader body 260 and a freely-openable original cover 252 attached to the reader body 260.
The reader body 260 includes a read unit 262 provided with reflecting mirrors 263, an image-forming lens 264, and a CCD 265. The read unit 262 is driven by an unillustrated driving belt and caused to travel on a reference shaft 266 in a subscanning direction. The reader body 260 also includes an original bed 261 for placing a translucent original 201 thereon.
The original cover 252 is configured such that a light source unit 254 having a light source 253 is driven by an unillustrated driving element and caused to travel synchronously with the travel of the read unit 262. The original cover 252 also includes a diffusion plate 255. The diffusion plate 255 is disposed so as to face the original bed 261 and is adapted to diffuse light from the light source 253 to thereby absorb a positional deviation of the light source 253 from the reflecting mirrors 263.
When the translucent original 201 is to be read, the translucent original 201 is placed on the original bed 261, and then the original cover 252 is closed. Subsequently, the translucent original 201 is irradiated with light from the light source 253. The light passes through the original bed 261 and reaches the CCD 265 via the reflecting mirrors 263 and the image-forming lens 264, to thereby form an image on the CCD 265. The thus-formed image of the translucent original 201 is converted to image data by the CCD 265.
FIG. 21 shows a detailed structure of the original cover 252. The original cover 252 has a guide rail 280 extending in the subscanning direction and serving as a driving element for driving the light source unit 254. A rack 281 is formed at one side of the guide rail 280. To a unit frame 279 of the light source unit 254 are attached the light source 253 and driving elements for the light source unit 254, such as a pulse motor 287 and gears 283 and 284. Further, the light source unit 254 has a pinion 282 serving as its driving element. The pinion 282 is journaled to be meshed with the rack 281. The pinion 282 is driven by the pulse motor 287 via the gears 283 and 284.
Two slide shoes 285 are provided on the light source unit 254 such that the slide shoes 285 are located on one side of the guide rail 280 opposite the rack-formed side thereof. The slide shoes 285 are pressed by means of pressing elements 286 so that the slide shoes 285 come in contact with the guide rail 280 at two positions located on opposite sides with respect to the meshing position between the rack 281 and the pinion 282. Thus, the two slide shoes 285 define the orientation of the light source unit 254.
Contact members 278 each formed of, for example, a sliding member are provided on opposite sides of the unit frame 279 of the light source unit 254 such that they abut the original cover 252. One contact member 278 presses a cover frame 256 of the original cover 252 via a pressing element 286. Thus, the light source unit 254 travels on the basis of the cover frame 256. The diffusion plate 255 is fixed on the cover frame 256 by means of, for example, screws.
The translucent original 201 to be scanned by the flatbed-type image reader assumes the following forms: a naked film 201a as shown in FIG. 22A; and the film 201a accommodated in a film folder or case 201b as shown in FIG. 22B. The film 201a is about 0.2 to 0.3 mm thick, and the film folder 201b is about 2 to 3 mm thick.
When the above-mentioned translucent original 201 placed on the original bed 261 is thick, the original cover 252 may be unable to be closed to a predetermined position. By contrast, when the translucent original 201 is thin, an improper assembling accuracy of an attachment portion of the original cover 252 may cause a failure to establish contact between the original cover 252 and the translucent original 201 in the vicinity of the attachment portion. In these cases, the distance between the light source 253 and the translucent original 201 varies along a main scanning direction. Also, when the original cover 252 is distorted in the Z direction of FIG. 20A, the distance (optical path length) between the light source 253 and the translucent original 201 varies (this distance is also an optical path length, whose definition is different from that of the previously mentioned optical path length). As a result, the quantity of light received by the CCD 265 is difficult to hold constant. Therefore, in order to read information from the translucent original 201 through movement of the light source 253, the attachment accuracy of the original cover 252 and the accuracy of a light-source-operating unit must be increased in order to make the quantity of light received by the CCD 265 constant.
Accordingly, the conventional flat-bed-type image reader capable of reading the translucent original 201 involves the following problems.
(4) When various kinds of translucent originals 201 having different thicknesses are to be read or when the original cover 252 is distorted, the distance (and as a result, the optical path length) between the light source 253 and the translucent original 201 varies, thus failing to maintain the quantity of light received by the CCD 265 at a constant level.
(5) The attachment accuracy of the original cover 252 and the accuracy of the light-source-operating unit must be high.
(6) Because of the above (4) and (5), there cannot be implemented an inexpensive mechanism capable of uniformly and stably reading an image.
An example structure of a conventional image reader having a function to read a translucent original formed of, for example, a transparent film will next be described with reference to FIG. 23. A light source unit 362 accommodated in an upper housing 365 has a light source 361. Light emitted from the fight source 361 passes through an original 375 placed on a transmission glass 373 provided on a top surface of a lower housing 374. The direction of the transmitted light is changed by a mirror provided in an optical unit 369 such that the light reaches a CCD 368 via a condenser lens 367.
The light source unit 362 is moved along the original 375 by means of a driving belt 370a, which is driven by a driving pulley 371a and a follower pulley 372a. Also, synchronously with the movement of the light source unit 362, the optical unit 369 is moved along the original 375 by means of a driving belt 370, which is driven by a driving pulley 371 and a follower pulley 372.
The relationship between the optical axis of light emitted from the light source 361 and the quantity of light received by the CCD 368 will next be described with reference to FIG. 25. According to the curve of FIG. 25, when the optical axis of light emitted from the light source 361 is aligned with the optical axis of the CCD 368, the optical axis of the light is positioned at C0, and the quantity of light detected by the CCD 368 becomes 100%; and when the optical axis of light emitted from the light source 361 deviates to a position C1, the quantity of light detected by the CCD 368 changes to K1%. The curve of FIG. 25 shows the following: there exists a region in the vicinity of the position CO in which the quantity of light received by the CCD 368 varies by a small amount with deviation of the optical axis of light. For example, when the optical axis of light deviates to the position C1, the received quantity of light varies to become K1%. By contrast, when deviation of the optical axis of light falls outside the above region of gentle variation, the quantity of light received by the CCD 368 drops greatly. For example, when the optical axis of light deviates greatly to position C4, the received quantity of light is varied to K3%.
Thus, in the image reader for reading a translucent original, in order to maintain high image quality without decreasing the quantity of light detected by the CCD 368, the following measures must be implemented, in addition to measures against the above-mentioned problems (4) to (6):
(7) To align the optical axis of light emitted from the light source 361 with the center of the CCD 368 of a light receiving unit.
(8) To prevent deviation of the optical axis of light emitted from the light source 361 during scanning of the original 375.
In order to align the optical axis of light emitted from the light source 361 with the center of the CCD 368 of the light receiving unit as mentioned above in (7), the conventional image reader is subjected to sufficient adjustment of its optical axis in a manufacturing process and is then shipped. In order to prevent deviation of the optical axis during scanning of an original as mentioned above in (8), the conventional image reader employs a pulse motor or a like device as a drive for enabling the optical unit 369 to perform scanning, thereby attaining accurate positional control during scanning. Further, the conventional image reader employs a pulse motor or a like device as a drive for enabling the light source unit 362 to perform scanning, thereby ensuring that the light source unit 362 follows the scanning motion of the optical unit 369 at sufficiently high accuracy.
Through employment of the above measures, there can be obtained an image reader having a practically sufficient optical-axis alignment. However, since a driving system including the driving pulley 371a, the follower pulley 372a, and the driving belt 370 must be accommodated within the upper housing 365, the upper housing 365 becomes large. Since an operator must open and close the upper housing 365 for scanning, an increase in the size of the upper housing 365 causes inconvenience to the operator.
As shown in FIG. 24, the optical unit 369 and the light source unit 362 may be integrated into a single unit to thereby fix the relative position between the optical unit 369 and the light source unit 362. However, in this case, opening and closing the upper housing 365 becomes difficult, and use of a reading mechanism which utilizes reflected light becomes difficult. As a result, the application of the image reader is significantly limited.
An image reader of the present invention reads an image through self advancement of a read unit provided with a reducing optical system employing an image-forming lens. The image reader comprises a read unit provided with a contact member which abuts an original placement bed of a flat bed unit, and a pressing element for causing the read unit and the original bed to press against each other.
An image reader of the present invention is of a flat-bed-type and comprises an apparatus body and an original cover. The apparatus body includes a read unit and an original bed. The original cover includes a light source and a diffusion plate and is attached to the apparatus body in such a manner as to be freely opened and closed. The distance between a translucent original placed on the original bed and the light source is variable according to the thickness of the translucent original.
The present invention provides a method for controlling an image reader including a light source unit and an optical unit. The light source unit has a light source for irradiating an original placed on a transmission glass with light. The optical unit receives light which has passed through the original. The method comprises the steps of previously reading a cumulative difference between the movement of the optical unit and the movement of the light source unit; and shifting, at the time of start of scanning for read, the relative position of the optical unit and the light source unit in reverse in relation to the direction of the cumulative difference so as to reduce an optical-axis deviation which arises during scanning.